The study addresses major public health concerns regarding the relationships between stimulant treatment in childhood on 1) incidence of SUD in later life, and 2) brain development. Based on developmental animal studies and brain imaging findings in individuals with ADHD, it is hypothesized that 1) age at first stimulant treatment is positively related to later SUD, and 2) there is an inverse relationship between age at first treatment and white matter volume, controlling for possible confounds (i.e., initial severity, SES, CD, IQ, etc.). The study capitalizes on a unique, large sample (N=226) of systematically diagnosed children with ADHD first treated with stimulants between ages 6-12 (M, 8y) (probands), and matched comparisons (N=188). We have detailed information on stimulant dose and treatment duration. Subjects have been followed up previously when at mean ages 18 (range: 16-23) and 25 (23-30), and have been tracked ever since. We propose: 1. to reevaluate probands and comparisons at mean age 39 (33-45), and spouses, for psychiatric history (SCID), detailed substance use (PRISM), IQ, social, educational, occupational, conjugal functioning, medical history. 2. to obtain high-resolution anatomic MRI and diffusion tensor imaging scans (on a Siemens Allegra 3.0 Tesla research-dedicated scanner). In addition to primary analyses, we will explore the relationship between features of stimulant treatment (duration, cumulative dose) and SUD, interactions between age at first stimulant treatment and MRI findings on SUD, and medical history. The study will also provide important information on adult functioning of children with ADHD. A key goal is to validate the relationship between earlier age at first treatment and lower risk for SUD by examining brain structure; importantly, even if the primary hypotheses are not confirmed, findings generated will have major significance by informing on possible sequelae of exposure to stimulants with regard to brain structure, medical liability, and will be the first to describe fully the functioning of children with ADHD in mid-adulthood, after age of risk for most disorders.